R help - Apr 2011 - Matching a vector with a matrix row

Hello I am trying to compare a vector with a Matrix's rows.The vector has
the same length as the number of columns of the matrix, and I would like to
find the row numbers where the matrix's row us the same as the given vector.
What I am doing at the moment is using apply as follows:

apply(Matrix,1,function(x)all(x%in%LHS))

but this isn't too fast actually. I would like  to know if any body knows an
efficient (fast) way of doing this? The matrix contains stings (not
numbers).

Thank you

Felipe Parra

	[[alternative HTML version deleted]]

Hi Felipe,

Since matrices are just a vector with dimensions, you could easily use
something like this (which at least on my system, is slightly faster):

results <- which(Matrix %in% LHS)

I'm not sure this is the fastest technique thought.  It will return a
vector of the positions in "Matrix" that match "LHS".  You
can easily
convert to row numbers if you want since all columns have the same
number of rows.

HTH,

Josh

On Thu, Apr 21, 2011 at 8:56 PM, Luis Felipe Parra
<felipe.parra at quantil.com.co> wrote:
> Hello I am trying to compare a vector with a Matrix's rows.The vector
has
> the same length as the number of columns of the matrix, and I would like to
> find the row numbers where the matrix's row us the same as the given
vector.
> What I am doing at the moment is using apply as follows:
>
> apply(Matrix,1,function(x)all(x%in%LHS))
>
> but this isn't too fast actually. I would like ?to know if any body
knows an
> efficient (fast) way of doing this? The matrix contains stings (not
> numbers).
>
> Thank you
>
> Felipe Parra
>
> ? ? ? ?[[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>


-- 
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
http://www.joshuawiley.com/

I gave a solution previously with integer elements.  It also works well for real
numbers.

rowMatch <- function(A,B) {
# Rows in A that match the rows in B
# The row indexes correspond to A
    f <- function(...) paste(..., sep=":")
   if(!is.matrix(B)) B <- matrix(B, 1, length(B))
    a <- do.call("f", as.data.frame(A))
    b <- do.call("f", as.data.frame(B))
    match(b, a)
}

A <- matrix(rnorm(100000), 5000, 20)
sel <- sample(1:nrow(A), size=100, replace=TRUE)
B <- A[sel,]

system.time(rows <- rowMatch(A, B ))
all.equal(sel, rows)

sel <- sample(1:nrow(A), size=1)
b <- c(A[sel,])
system.time(row <- rowMatch(A, b))
all.equal(sel, row)

I am curious to see if there are better/faster ways to do this.

Ravi.
________________________________________
From: r-help-bounces at r-project.org [r-help-bounces at r-project.org] On
Behalf Of Petr Savicky [savicky at praha1.ff.cuni.cz]
Sent: Sunday, April 24, 2011 5:13 AM
To: r-help at r-project.org
Subject: Re: [R] Matching a vector with a matrix row

On Sat, Apr 23, 2011 at 08:56:33AM +0800, Luis Felipe Parra
wrote:
> Hello Niels, I am trying to find the rows in Matrix which contain all of
the
> elements in LHS.
This sounds like you want an equivalent of

  all(LHS %in% x)

However, in your original post, you used

  all(x %in% LHS)

What is correct?

If the equality of x and LHS should be tested, then try

   setequal(x, LHS)

If the rows may contain repeated elements and the number of
repetitions should also match, then try

  identical(sort(x), sort(LHS))

with a precomputed sort(LHS) for efficiency.

If the number of the different character values in the whole
matrix is not too large, then efficiency of the comparison
may be improved, if the matrix is converted to a matrix
consisting of integer codes instead of the original character
values. See ?factor for the meaning of "integer codes".
After this conversion, the comparison can be done by comparing
integers instead of character values, which is faster.

Hope this helps.

Petr Savicky.

______________________________________________
R-help at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.